This invention relates to olefin isomerization. In one of its more specific aspects, this invention relates to selective isomerization of olefins using binder ferrierite zeolites and the synergistic effects of binders on zeolite catalysts used in such isomerization.
More particularly, the present invention relates to a process for the preparation of useful hydrocarbons by binder catalytic conversion of n-olefins.
MTBE (methyl tertiary butyl ether) is an effective octane booster. It is made from isobutylene and methanol. The present sources of isobutylene for MTBE production are mainly from by-products of steam, catalytic crackers, and propylene oxide production. However, these supplies are limited. Other possible sources are by isomerization of n-butenes taken from steam or catalytic crackers and by dehydrogenation of isobutane taken from field butanes or produced by isomerization of n-butane.
Olefin isomerization processes can be directed towards either skeletal isomerization or double bond isomerization. Skeletal isomerization is concerned with reorientation of the molecular structure in respect to the formation or elimination of side chains. Double bond isomerization is concerned with relocation of the double bond between carbon atoms while maintaining the backbone of the carbon structure. Most isomerization processes give rise only to double bond isomerization.
The minimum Bransted Acid strengths (and equivalents in H.sub.2 SO.sub.4) required for various acid-catalyzed conversions of hydrocarbons are indicated in the table below.
______________________________________ Minimum Bronsted Acid Strength Required For The Acid-Catalyzed Conversions of Hydrocarbons H.sub.R Required Reaction Type ______________________________________ &lt;+0.8 Cis-trans Isomerization of 1.2 wt % H.sub.2 SO.sub.4 Olefins &lt;-6.6 Double-bond Migration 48 wt % H.sub.2 SO.sub.4 &lt;-11.6 Skeletal Isomerization 68 wt % H.sub.2 SO.sub.4 &lt;-16.0 Cracking of Alkanes 88 wt % H.sub.2 SO.sub.4 ______________________________________
It is frequently necessary to convert olefins into other olefins having a different skeletal arrangement. For example, normal butenes are converted into isobutene for polymerization, alkylation, disproportionation or for the production of MTBE. Similarly, normal amylenes must be converted to isoamylenes prior to dehydrogenation to isoprene.
While a number of catalytic materials possess some activity for such a conversion, not all possess sufficient selectivity to be economical. Because the feeds are generally the relatively reactive olefins, many catalysts cause undesirable side reactions such as polymerization or cracking. Consequently, there is a continuing interest in the development of new skeletal isomerization catalysts and processes for isomerizing alkenes to improve efficiencies and to give optimum results for various industrial requirements. A comprehensive review is provided by V. R. Choudhary in "Catalytic Isomerization of n-butene to Isobutene," Chem. Ind. Dev, pp. 32-4 (1974).
It is generally known that n-paraffins with, for example, 4 to 7 carbon atoms can be converted to the corresponding isomeric paraffins by using suitable acid catalysts in the temperature range of from 100.degree. to 250.degree. C. Examples of this process are the numerous isomerization processes used in the petrochemical and mineral oil industries for increasing the octane number of light, paraffinic mineral oil fractions. Furthermore, it is known that, in contrast to this, olefins of the same number of carbon atoms cannot be converted to the corresponding isoolefins except under difficult conditions, for example at very high temperatures and with poor yield. The attempts hitherto described in the literature for the direct isomerization of the skeleton of e.g. n-butene to give isobutene or e.g. of n-pentene to give isopentenes over catalysts arranged in a fixed bed are characterized by initially relatively low yields and selectivities, which diminish and deteriorate further after a short period of operation, often after only a few hours. The deterioration in the yields and selectivities is generally attributed to the loss of actively effective catalyst surface or to the loss of active centers. In addition to this, high coking rates, formation of oligomers and cracking reactions are observed.
As is known, olefins exist in various isomer. for example, butylenes or butenes exist in four isomers: butene-1, cis-butene-2, its stereo-isomer trans-butene-2, and isobutene; and pentenes exist in six isomers. Conversions between the butenes-2 are known as geometric isomerization, whereas those between butene-1 and the butenes-2 are known variously as position isomerization, double-bond migration, or hydrogen-shift isomerization. These three isomers are not branched and are known collectively as normal or n-butenes. Conversion of the n-butenes to isobutene, which is a branched isomer, is widely known as skeletal isomerization.
Similar, conversions between the 2-pentenes are known as geometric isomeration, whereas those between 1-pentene and the 2-pentenes are known variously as position isomerization, double-bond migration, or hydro-shift isomerization.
Olefins, such as isoamylenes or isobutene have become more and more important recently as one of the main raw materials used in the production of methyl tert-butyl ether (MTBE), an environmentally-approved octane booster to which more and more refiners are turning as metallic additives are phased out of gasoline production. However, processes for the skeletal isomerization of olefins e.g., to produce isobutene, are relatively non-selective, inefficient, and short-lived because of the unsaturated nature of these compounds. On the other hand, positional and skeletal isomerization of paraffins and alkyl aromatics are fairly well established processes, in general utilizing catalysts typically comprising metallic components and acidic components, under substantial hydrogen pressure. Since paraffins and aromatics are stable compounds, these processes are quite successful. The heavier the compounds, in fact, the less severe the operating requirements. Olefins, however, are relatively unstable compounds. Under hydrogen pressure, they are readily saturated to the paraffinic state if a metal component is present in the catalyst.
Furthermore, in the presence of acidity, olefins can polymerize, crack and/or transfer hydrogen. Extensive polymerization would result in poor yields, and short operating cycles. Similarly, cracking would reduce yield. Hydrogen transfer would result in saturated and highly unsaturated compounds, the latter being the common precursors for gum and coke. Any theoretical one step process for producing skeletal isomers of, for example, n-butenes or amylenes, would have to be concerned with the unwanted production of olefin oligomers and cracked products. In addition to these problems, it is well known that skeletal isomerization becomes more difficult as hydrocarbons get lighter.
Skeletal isomerization of olefins is known to be accomplished by contacting unbranched or lightly branched olefins with acidic catalysts at elevated temperatures. The process is generally applicable to the isomerization of olefins having from 4 to about 20 carbon atoms and is especially applicable to olefins having from 4 to about 10 carbon atoms per molecule. The process may be used to form isobutene from normal butylenes, methyl pentenes and dimethyl butenes from normal hexenes, and so forth.
In making the isomerization of these olefins, the zeolite crystallites, i.e., catalysts, are usually bound together within a matrix or binder generally comprised of alumina, silica, silica-alumina, clay or admixtures thereof to enhance the performance of the zeolite catalysts (e.g., the yield of product).
Thus, among the objects of this invention are improved processes for the skeletal isomerization of n-butylene and olefins, especially for the isomerization of n-butylene to form isobutylene with zeolite catalysts to enhance the performance bound of the catalysts.
Other objects and advantages of the invention will be apparent from the following description, including the drawing and the appended claims.